Presynaptic reactions were identified which regulate the number of NBr10-A synapses with striated muscle cells and the efficiency of trans-synaptic communication. Synapse plasticity was regulated by the acquisition of functional voltage-sensitive Ca2 plus channels by NBr10-A cells. Monoclonal antibodies were used as probes to detect molecules needed for synaptogenesis or transynaptic communication. Antibodies were obtained which either affect the frequency of miniature end-plate potentials at synapses, recognize certain cell types in the nervous system, or exhibit regional specificity in the nervous system. An antibody with topographic specificity also was obtained which recognizes cell surface molecules that are distributed in a dorsal yields ventral concentration gradient in avian retina. The gradient of antigen can be used to define the dorsoposterior yields ventroanterior axis of the retina and the relative positions of cells in retina along this axis.